1. Field of the Invention
This invention relates to pistons for internal combustion engines, and more particularly to a wrist pin as used in piston assemblies to interconnect the piston body to a connecting rod.
2. Related Art
Piston assemblies as used in internal combustion engines are subject in use to extremely high temperatures, forces and inertia. Optimal operation of such engines requires careful management of the temperature of the components of a piston assembly which are typically manufactured from strong, lightweight metal alloys.
A practice long observed in diesel engine applications is to circulate lubricating oil through passages in the piston assembly to serve as both lubricant for the various joints and as a cooling medium to draw heat away from the piston crown. U.S. Pat. No. 2,742,883 to Smith, issued Apr. 24, 1956, disclose one such piston assembly wherein cooling oil is ported through a hollow wrist pin and then moved into an upper gallery formed just under the crown of the piston head.
In another example, U.S. Pat. No. 3,702,092 to Zollner, issued Nov. 7, 1972, discloses a lightweight wrist pin assembly formed with a plurality of open air channels or passages extending completely therethrough. The passages may be circular or have other shapes and range in number from two to six. In certain examples, the passages do not extend completely through but are internally stopped and open only at one end of the wrist pin. The stopped holes or through passages in the wrist pin are created in this case for the purpose of weight reduction.
In yet another example, U.S. Pat. No. 4,005,686 to Wizemann et al., issued Feb. 1, 1997 discloses a piston assembly including a wrist pin having a hollow passage therein open to receive pressurized cooling liquid (i.e., lubricating oil) from the connecting rod. The oil is routed through the wrist pin and into the piston head or crown area.
It has been observed that, at least in diesel applications, pistons operating under a thermal load equal to or higher than 0.4 kW/cm2 generally require forced piston cooling in order to maintain temperatures below 270° C. At least 2 kW is thus required in a 6-cylinder heavy duty truck engine and considerably more as the size and/or specific power (kW/liter) of the engine increases. This forced piston cooling represents a parasitic loss. That is, fuel is burned in order to move this cooling liquid without any corresponding useful power being developed.
There is therefore a need in the art to provide more effective and efficient means by which to rid the crown of a piston of absorbed heat, for the purpose of reducing or eliminating entirely the losses associated with forced cooling. Improved methods are thus sought after with the goal of recovering parasitic losses inherent in prior art engine systems.